For certain applications such as the document security field the aim involved is to offer security features which can be easily perceived and explained and which are defined for recognizing authenticity.
For example WO 97/19821 describes an optically variable surface pattern having at least one representation of a graphic configuration. Surface regions of a given kind are associated with each representation. Upon illumination with visible light the representations become visible in the form of images which exhibit light and dark spots at different viewing angles. The above-mentioned surface regions have grating structures, the line number of which is greater than 250 lines/mm so that markedly visible diffraction effects occur in the visible range.
A document with an optical-diffraction security element is known for example from EP 0 105 099 B1. The security element has a diffraction structure which, by virtue of diffraction of the incident light, produces at least one color pattern which represents a visually checkable authenticity feature. The diffraction structure is such that the color pattern which occurs with a given lighting and viewing direction moves at a locally predetermined speed along a predetermined path when the substrate is turned in its plane in a given direction of rotation and at a given speed.
EP 0 375 833 B1 discloses an optically variable surface pattern with a graphic representation, the surface pattern being divided into m-raster fields which contain optically active diffraction elements with more than 10 lines/mm. That known optically variable surface pattern provides that each raster field has a largest dimension of less than 0.3 mm and division into N-field components each with a respective diffraction element. Each diffraction element corresponds to a predetermined pixel of one of N-representations. The parameters of a relief structure are determined in each of the N-diffraction elements by the association with a surface portion so that each of the N-representations is visible in a predetermined direction of viewing the surface pattern.
U.S. Pat. No. 3,858,977 describes an optical interference security element. That known element has a substrate and a filter having at least one optical interference layer. The optical interference layer has a known characteristic in respect of its spectral reflectance and a known spectral transmittance which differs therefrom, both varying with the angle of incidence of light. The substrate which can form a surface region of a value-bearing document such as a banknote has a surface portion which is adjacent to the filter and is colored in order to absorb a part of the light transmitted through the filter.
Optically variable surface patterns which are designed like the above-described optically variable surface pattern, depending on the respective choice of design and technology, can produce different visual, optically variable effects, in dependence on the viewing angle, such as three-dimensionality or perspectivity, motion effects, image flips or the like. When the surface pattern or the element provided with such a surface pattern is turned or tilted about given axes in space, many design elements appear in the typical rainbow colors.
That virtually continuous transition over the entire rainbow spectrum now suffers from disadvantages in some security uses. It is in conflict with the aim of providing security features which can be easily recognized and explained and which are as well-defined as possible, for authenticity recognition. More specifically such a continuous color transition or color change represents a definition problem. In addition, a clear demarcation of features in respect of functionality and appearance between security application and commercial applications is necessary in order to prevent confusions occurring due to simple imitation or due to the transfer of commercial features onto security documents and so forth.